This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A093946 #18 Jun 14 2025 23:47:57
%S A093946 5,59,599,59999,599999,59999999,59999999999,59999999999999,
%T A093946 59999999999999999999999,599999999999999999999999,
%U A093946 59999999999999999999999999999,59999999999999999999999999999999999
%N A093946 Primes of the form 6*10^k - 1.
%C A093946 Equivalently, primes of the form 5*10^n + 9*R_n, where R_n is the repunit (A002275) of length n.
%H A093946 Makoto Kamada, <a href="https://stdkmd.net/nrr/5/59999.htm#prime">Prime numbers of the form 599...99</a>.
%H A093946 <a href="/index/Pri#Pri_rep">Index entries for primes involving repunits</a>.
%F A093946 a(n) = 6*10^A056716(n) - 1 = A099151(A056716(n) + 1). - _Elmo R. Oliveira_, Jun 14 2025
%t A093946 Select[Table[FromDigits[PadRight[{5},n,9]],{n,40}],PrimeQ] (* _Harvey P. Dale_, Jun 06 2016 *)
%o A093946 (Magma) [a: n in [0..200] | IsPrime(a) where a is (6*10^n-1)]; // _Vincenzo Librandi_, May 08 2019
%Y A093946 Cf. A002275, A056716.
%Y A093946 Primes in A099151.
%K A093946 nonn
%O A093946 1,1
%A A093946 _Rick L. Shepherd_, Apr 17 2004